The present invention relates generally to the field of fluid flow indicators. More particularly, the present invention relates to a gas flow indicator which utilizes a highly sensitive gas flow switch in combination with a timer, alarm system and optional activity sensor for alerting the presence of sustained gas flow after a predetermined period of time.
Gas-operated appliances, such as barbecue grills, deck heaters, fireplaces, and the like have become commonplace. The availability of either natural or propane gas as a fuel makes these appliances particularly convenient. However, it is commonplace for the users of such appliances to accidentally leave the appliances turned on long after their intended period of use. Portable propane tanks are frequently exhausted after the users forget to turn the appliances off. This leads to an expensive waste of fuel, the inconvenience of refilling portable gas tanks, and a considerable safety hazard. The problem is magnified when the appliances are plumbed directly to the available natural gas supply or household propane tank. With millions of gas grills and other gas appliances being sold around the world each year, clearly an effective signaling/safety device is desirable.
Others have heretofore attempted to solve the above problem. Nearly all such attempts, however, have resulted in devices in which the intended purpose is to turn off the appliance (cease gas flow) after a certain predetermined amount of time. Patents disclosing this type of a gas flow control device include the following: U.S. Pat. Nos. 6,684,757; 5,628,242; 4,823,838; 6,733,276; 5,813,394; 6,289,792; 5,617,840; 7,117,893; and 5,333,596. To the knowledge of the applicant, however, none of these devices have ever become standard equipment on any commercially available gas appliance.
The reason for this is due primarily to the increased liability associated with such devices. Once a device of this nature is installed, there becomes an assumption of safety on the part of the owner, where he/she begins to rely on such a device as a convenience feature designed to automatically turn off the appliance. While it may indeed be convenient, should the device fail for any reason, the device which was originally intended as a safety feature, is now regarded as the cause of something potentially catastrophic.
In such devices, a single “upstream” valve is virtually always disclosed as a master valve to cut off the supply of gas to the burners. However, the individual burner valves will still remain in their previously open position, allowing even small amounts of gas to pass in the event the master cutoff valve fails to seal completely. This accumulated gas could lead to a catastrophic explosion the next time the appliance is operated. Therefore, gas-operated appliance manufacturers have been reluctant to include such “automatic” devices on their products, preferring instead to rely on the user to turn off the device himself, effectively leaving responsibility, and therefore liability, with the user.
In U.S. Pat. No. 5,608,383, an automatic temperature alarm system is disclosed, wherein the device is more appropriately intended to be a warning device rather than a shut-off switch. This device utilizes a temperature sensor to detect operation of a gas grill and a vibration sensor to detect whether the grill has been untouched for an excessive period of time, thereby sounding the alarm. Any detection of vibration causes a timer to reset, thus preventing the alarm from sounding.
Regardless of the type of system utilized, reliable detection of appliance operation is paramount. Unfortunately, with today's more sophisticated gas appliances, temperature is not the best means for detecting operation. In older gas grills, for instance, where there may have been only one or two burners, both housed within a single grill chamber, a single temperature sensor may have been sufficient. But with today's grills, multiple burners, as well as accessory side burners and dedicated rotisserie burners, are common. This would require multiple temperature sensors to accurately determine whether or not the grill is actually operating, complicating installation and raising the cost of implementation.
Other detection mechanisms have also been previously contemplated. The detection of gas valve/knob position would only indicate an intent to use, and all valves/knobs would need to be individually instrumented. Flame detection and non-contact infra-red suffer from the same multi-burner issues as the temperature sensor. External non-contact infrared also suffers from issues of “field of view”, sensor contamination, and varying surface properties. Additionally, calibration is required for the property known as emissivity, making the same system difficult to use on appliances constructed of different outer materials, such as porcelain enamel versus stainless steel.
A more viable and safer alternative for detecting operation of a gas appliance is to sense gas flow to the appliance. Detecting gas flow has the advantage in that it can reliably cover all burners, whether ignited or not, from a single location. However, sensing the flow of gas in a gas grill is complicated by the fact that both the pressure and the flow rate of gas to the grill are extremely low. For instance, in a typical propane gas grill, high pressure liquefied propane is stored in a portable tank attached to the grill. The pressure in the tank varies with temperature, but may well be over 150 psi. Because of this, a low pressure regulator is required to reduce the pressure of the gas to a safe working pressure of just 11″ water column (WC), which is just 0.4 psi. For natural gas systems, the pressure is even lower, typically just 7″ WC, or 0.25 psi. In the case of propane, the rate of flow on the high pressure side of the regulator, due to the increased pressure, is even lower than that on the low pressure side. This makes reliable detection of the flow of gas difficult.
Gas flow switches/detectors are available in the prior art, but most are ill-suited for use in low pressure/low flow applications. They are available in many forms, but typically employ a piston or a vane, which activates an electrical switch. Piston designs can be used in what is known as “positive displacement” configurations. While a typical gas flow switch may indicate when the flow rate is above a particular threshold, a positive displacement design ideally does not allow gas to flow at all without providing an indication.
In such a device, a differential pressure generated within the switch body between the input and the output provides the necessary operating force to move the piston out of the way, allowing gas to flow. Typically, a magnet residing within the piston is used to trigger an external reed switch or Hall effect sensor attached to the flow switch body, thereby detecting the flow of gas. A spring, magnet, or gravity is generally used to provide the return force necessary to reset the gas flow switch in the absence of gas flow.
U.S. Pat. No. 6,684,757 mentions in passing the possibility of detecting operation of a gas grill via the detection of gas flow, but it fails to address the problem and difficulty of sensing gas flow in such a low pressure/low flow application as is present with the gas grill appliance. For such low pressure applications, where the gas flow switch may be placed downstream of the pressure regulator, both the flow and the pressure are too low for a typical flow switch. Such switches tend to reduce pressure and restrict flow, thus hindering the performance of the appliance. Furthermore, whether positioned downstream or upstream from the pressure regulator, pistons utilized in such gas flow switches are commonly subject to oscillation. Therefore, enhancements must be made in order for reliable gas flow detection to function without significant pressure loss and flow restriction.
It is therefore an object of the present invention to provide a novel configuration of a fluid flow indicator/switch which is highly sensitive and useful for detecting the flow of gas in low flow applications, such as with gas grills and the like, and which will function accurately without significant pressure loss or flow restriction.
It is also an object of the present invention to provide a novel configuration of a piston-type positive displacement fluid flow indicator/switch which is highly sensitive and useful for detecting the flow of gas in low flow applications, such as with gas grills and the like, and which is devoid of any substantial piston oscillation.
It is still a further object of the present invention to provide such a novel and useful fluid flow indicator which utilizes a highly sensitive fluid flow switch in combination with a timer, alarm system and optional activity sensor for alerting the presence of sustained gas flow after a predetermined period of time.